In recent years, a dye-sensitized solar cell is proposed by Graetzel et al. (see National Publication No. H05-504023). This dye-sensitized solar cell can be fabricated by a simple process and may be constituted of inexpensive materials. Accordingly, there is a high possibility that it can be obtained at a lower cost than conventional silicon type solar cells, and studies thereon are energetically made toward the achievement of its practical use.
The dye-sensitized solar cell is constituted of a transparent electrode formed on a transparent substrate, an oxide semiconductor electrode formed on the transparent electrode, a dye adsorbed on the oxide semiconductor electrode, an electrolyte, and an opposing electrode. The electrolyte is filled between the dye adsorbed oxide semiconductor electrode and the opposing electrode provided opposingly thereto. Upon irradiation of the dye adsorbed oxide semiconductor electrode with visible light such as sunlight, a potential difference is produced between the oxide semiconductor electrode and the opposing electrode to make electric current flow across both the electrodes.
As the transparent electrode of the dye-sensitized solar cell, commonly used are indium tin oxide (ITO) formed by sputtering or the like and fluoro-tin oxide (FTO) formed by chemical vapor deposition (CVD). Also, the oxide semiconductor electrode is constituted of fine titanium oxide particles, and a ruthenium (Ru)-based dye such as a ruthenium bipyridyl complex is used as the dye. Platinum (Pt), carbon or the like is used in the opposing electrode. As the electrolyte, used is an iodide-based electrolyte prepared by dissolving iodide (I2) or lithium iodide (LiI) in an organic solvent such as acetonitrile, ethylene carbonate, propylene carbonate or polyethylene glycol (PEG).
Now, the above conventional transparent electrode is formed by a physical film forming process such as sputtering or CVD, and has a film surface resistivity of about 10 Ω/square. The transparent electrode having such a degree of film surface resistivity is well usable in a dye-sensitized solar cell having a size of about several millimeters square. However, if the dye-sensitized solar cell is one having a size of 10 cm square or more than that, the electric current is consumed as Joule heat at the transparent electrode portion, and hence this lowers electricity generation efficiency extremely.
Therefore, in order to make the dye-sensitized solar cell have a size large enough to be of practical use, it is necessary to make the transparent electrode have a vastly low film surface resistivity, i.e., at least a film surface resistivity of about 1 Ω/square or less. For this end, the transparent electrode may merely be made to have a large layer thickness to lower its film surface resistivity, for example. In such a case, however, because of the large layer thickness, the transparent electrode has a vastly low light transmittance and also a low electricity generation efficiency, and hence it is not practical to do so.
Accordingly, as shown in FIG. 1, for example an auxiliary electrode layer 3 composed of a metallic component may be formed in a pattern on a transparent electrode layer 2 formed on a transparent substrate 5, to lower the film surface resistivity. Such a method is known in the art. As materials for such an auxiliary electrode layer 3, silver and copper are suitable as having a low resistivity. In the case of the dye-sensitized solar cell, however, the iodide-based electrolyte used as the electrolyte is very highly corrosive, and hence, not to speak of silver and copper, even gold is not usable. Also, in order to prevent this auxiliary electrode layer from corroding, a method is also proposed in which an auxiliary electrode layer on a transparent conductive layer is covered with a protective thin film formed of tin oxide, titanium oxide or the like (see Japanese Patent Application Laid-open No. 2003-203683).
However, such an auxiliary electrode layer must be formed in a layer thickness of several microns to tens of microns (μm) in order for that layer to achieve the intended function, and hence it follows that unevenness (hills and dales) of several microns to tens of microns (μm) in extent comes about on the side where devices are to be formed on the transparent conductive layer. This brings about great restrictions on the formation of devices (e.g., devices must be formed only in areas where any pattern-shaped auxiliary electrode layer is not formed). Also, the upper limit of the thickness of the auxiliary electrode layer depends on the device structure, and is limited to a stated value or less (e.g., 20 μm or less in the case of the dye-sensitized solar cell). Hence, there has also been a limit to how the transparent electrode is made to have a low film surface resistivity. Moreover, in the case when the auxiliary electrode layer is covered with the protective thin film, the thickness of the protective thin film must be controlled to be about 50 nm or less in order to control the protective thin film to have a film surface resistivity of a stated value or less to make electric current flow to the auxiliary electrode layer though the protective thin film. Hence, it has been difficult to achieve a sufficient protective effect without making devices have poor characteristics.
Besides such a dye-sensitized solar cell, as a device required to have a low-resistance transparent electrode, an organic electroluminescent device (hereinafter “organic EL device) is available which is considered promising for its use in display, illumination and so forth. The organic EL device is a self-light-emitting device comprising an ITO or the like transparent electrode and multi-layered thereon a hole injection layer, a polymer light-emitting layer, a cathode layer and so forth. It is not a voltage drive type device such as a liquid-crystal device, but a current drive type devices, and hence, in order to make the device have a large size, it is essential to make its transparent electrode have a vastly low film surface resistivity.
Accordingly, like the above dye-sensitized solar cell, a method is available in which an auxiliary electrode layer is formed in a pattern on the transparent electrode layer to lower the film surface resistivity. In this case as well, however, like the case of the dye-sensitized solar cell, there has been a problem that the unevenness due to the formation of such an auxiliary electrode layer brings about great restrictions in regard to the fabrication of devices.
Moreover, conventional transparent conductive layers used in the dye-sensitized solar cell and organic EL device are formed by film forming processes such as sputtering and CVD, which require large-size and expensive systems. Hence, there have been problems that a very high cost results and besides it is difficult to form transparent conductive layers on transparent substrates having a poor heat resistance, such as plastic films.
Incidentally, in place of the transparent conductive layer forming methods such as sputtering and CVD, a method (coating method) is also proposed in which a plastic film is coated thereon with a transparent conductive layer forming coating fluid containing fine ITO particles dispersed therein, to form a transparent conductive layer. However, the transparent conductive layer thus obtained has so high a film surface resistivity that it can not be said to be practical for its use in the devices such as the dye-sensitized solar cell. As a method by which the transparent conductive layer obtained by this coating method is improved in conductive properties, a method is further proposed in which the film is coated thereon with the transparent conductive layer forming coating fluid, followed by drying and thereafter rolling by means of steel rolls or the like (see Japanese Patent Application Laid-open No. H04-237909). However, the transparent conductive layer obtained has a film surface resistivity of about hundreds of Ω/square, which is still insufficient.